1. Field of the Invention
This invention relates to packed tower or packed column structures used for intimate contact between a gas and a liquid to effect mass transfer therebetween. More particularly, this invention relates to a mobile packing material arrangement for a packed tower that provides unique advantages when the method of using the tower includes cocurrent downflow contact between gas and liquid, and provides new and unexpected results in cleaning the packing material. The method of contacting gas and liquid downwardly over a bed of mobile packing material provides intimate contact between gas and liquid, and is particularly useful for gas-liquid mass transfer in relatively large absorption towers having a cross-sectional area more than about 3 ft.sup.2, where gas channeling, and surging of gas and packing material is most prevalent using counterconnect flow of gas and liquid. This invention is particularly useful in gas-liquid contacting processes where the packed tower or column is prone to undergo plugging due to the formation of solid material as a result of the gas-liquid contacting process.
2. Related Technology
It is well known that packed towers or columns are often useful for efficient gas-liquid contact to provide interactions, reactions and other mass transfer operations between gas and liquid. However, in some such operations the fluid passing through the packed bed contains suspended solid particles that tend to accumulate on the packing, eventually resulting in a reduction in gas-liquid volumetric flow rates and, in extreme cases, plugging of the tower. In particular, when such solids are being formed by chemical means within the tower, such as by crystalization of salts from solution or by precipitation of sulfur by oxidation of soluble surfides, the newly formed solid particles are prone to deposition on any surfaces with which they come into contact.
Such plugging problems occur, for example, when a packed tower is used with liquids or gases that contain particulate material, or when particulates are formed within the tower as a result of a chemical interaction, reaction or the like, that precipitates solids as a result of mass transfer between the liquid and the gas. For example, in flue gas scrubbing that utilizes a liquid stream that includes an aqueous solution or suspension of limestone or the like, to remove sulfur dioxide contained in a gas stream, a calcium sulfate precipitate is formed in the tower, and the resulting aqueous slurry causes plugging problems. Another example is a gas-liquid interphase oxidation process for removal of hydrogen sulfide from a gas stream in which the scrubbing liquid is an oxidizing solution that effects oxidation of hydrogen sulfide to form elemental sulfur particles. Such particles may cause plugging of packed towers, requiring periodic shutdown of the process for a difficult and time consuming cleaning of the tower.
Some prior processes used packed towers for contact of hydrogen sulfide (H.sub.2 S)-containing gases with an aqueous iron-chelate solution or a so-called "Stretford"-type alkaline vanadium ion-containing wash solution that converts H.sub.2 S to elemental sulfur (S.degree.). The elemental sulfur formed is entrained in the liquid mixture, and removed from the liquid, e.g. by settling or filtration. In this type of H.sub.2 S removal process, some of the sulfur settles on the packing material, thereby lowering gas and liquid flow rates and, ultimately, the sulfur plugs the tower. Mobile packing beds have been used in packed towers for sulfur removal from H.sub.2 S-containing gas streams but suffer from uneven gas distribution and channeling problems, particularly in larger cross-sectional area towers, resulting in decreased efficiency.
It is well known that two of the most important characteristics necessary for effective and efficient gas-liquid mass transfer in packed towers are: 1) the tower must contain adequate passages for both the liquid and the gas streams without excessive liquid holdup or pressure drop; and 2) the tower must provide good contact between liquid and gas. The requirement of good contact between liquid and gas is the most difficult to meet, especially in large towers. Ideally, the liquid, once distributed over the top of the packing, flows in thin films over all the packing surface completely down the tower. Actually, the films tend to grow thicker in some places and thinner in others, so that the liquid collects into small rivulets and flows along localized paths through the packing. Especially at low liquid rates, and in the larger towers, some of the packing surface may be dry or covered with a stagnant film of liquid. This effect is known as "channeling" and is the main reason for poor performance in large packed towers. Similarly, a countercurrently flowing gas stream in larger towers sometimes tends to follow localized paths instead of being distributed across the entire cross-section of the tower, with a consequent increase in velocity through these localized paths, and this effect is known as "surging".
The use of mobile packing beds to decrease or eliminate packing material fouling adds to the "channeling" and "surging" problems inherent in packed towers of relatively large cross-sectional area due to the tendency of the relatively light mobile packing material to collect along the sides and other localized areas of the tower, further decreasing the desired cross-sectional uniformity of gas and liquid flow. This low efficiency problem in relatively large mobile bed gas-liquid contact devices has been recognized for many years but, to date, no adequate solution to the channeling and surging problems has been achieved. Others have provided vertical partitions in the columns between grids, to provide a plurality of partitioned packing beds between grids, thereby decreasing the cross-sectional area of each separate packing bed. This partitioning has aided the gas and liquid flow uniformity, but not completely satisfactorily, and has created other problems, such as the attendant difficulty in loading and unloading packing material and making sure that loading is done uniformly within each separate compartment.
When plugging occurs in a packed column of fixed packing material, the cleaning procedure is not easy to accomplish and is sometimes ineffective. The best method of cleaning which has been developed to date involves:
a) Shutting off the gas flow;
b) Closing the liquid drain valves at the bottom of the vessel;
c) Filling the tower with water or other aqueous cleaning solution to a level above the top of the fixed packing;
d) Bubbling gas through the vessel at a relatively high rate (.apprxeq.20 ACFM/ft.sup.2) to achieve a lifting and agitation of the packing to free the sulfur;
e) Allowing the sulfur to settle through the cleansing solution and the packing into the bottom of the scrubber,
f) Repeating d) and e) several times,
g) Draining the liquid out of the scrubber; and
h) Returning the scrubber to the normal service.
This procedure usually cleans the packing elements sufficiently for continued operation but, if not, the alternative involves mechanically removing the packing and washing it with water, which is very time-consuming and costly.
As an alternative to fixed packing elements, others have used a Mobile Bed Absorber (MBA) technique, using hollow plastic spheres of about the size and shape of ping-pong balls to replace the fixed packing in counterflow columns. This MBA technique has been used successfully in a number of applications, particularly in small columns. While this MBA countercurrent flow approach has proven satisfactory for small diameter towers there have been serious difficulties in larger diameter columns; i.e., those over about 2' in diamter. While the spherical hollow packing elements are almost always maintained in a sufficiently agitated state to prevent fouling with sulfur, high efficiency operation cannot be obtained unless the distribution of gas and liquid flow across the cross-section of the tower is very uniform. This uniformity becomes extremely difficult to obtain as the horizontal dimension of the scrubber approaches the vertical depth of the packing material when not aerated (static).
This uneven cross-sectional distribution of gas and liquid has caused serious problems in some large commercial units. Partitioning the fluidized bed into a number of small, parallel counterflow absorbers aids in providing more uniform gas and liquid flow, but achieving uniform gas flow within the compartments remains a problem, even with compartmentalization of stages. Also, in large cross-section towers, e.g. 12'.times.15' horizontal dimensions, 20 compartments measuring 3 feet by 3 feet are required, and the logistics of loading and unloading the spherical packing are difficult.
The present invention solves this gas-liquid flow problem within mobile bed packed towers, without vertically partitioning the mobile beds, by providing a method that uses cocurrent downward flow of gas and liquid over spherical mobile packing material to provide good mass transfer efficiency at an unexpectedly low pressure drop, comparing favorably with random, dumped packings, and having unexpectedly even liquid and gas flow without channeling and surging problems. The spherical shape of the light, mobile packing elements have some rotational motion during cocurrent downflow mass transfer to provide some self-cleaning and a decreased tendency to plug the tower. Further, the spherical packing shape eliminates corners and crevices between packing elements to decrease build-up of solids between packing elements. Most unexpectedly, the spherical mobile packing elements can be cleaned easily on a periodic basis, in a new, more effective and simpler manner since the spheres float in water during the cleaning cycle to separate the packing elements from the settled solids with little or no upward gas bubbling required thorugh the beds of packing material, thereby providing a liquid-filled volume for particulate settling without interference from packing material; and the spherical shape of the packing elements makes them less subject to recapture of particulates.